Touch scrolling stops working in the main window if a separate window is shown on loading in a separate thread

# Touch scrolling stops working in the main window if a separate window is shown on loading in a separate thread

If we show a separate window with some additional details (a kind of Splash Screen) on loading in a separate thread, touch scrolling stops working in the main window. This issue occurs only in WPF NET Core. When we use the same code in WPF .NET Framework everything operates correctly. You can see this difference in the projects and screencast that I attached below.

Thank you in advance.
Andrey

[dxSample.zip](https://github.com/dotnet/core/files/5390191/dxSample.zip)
[screencast (MP4).zip](https://github.com/dotnet/core/files/5390211/screencast.MP4.zip)